Media, in the sense of the present invention, are frequently employed, particularly in drive technology, as lubricants and/or coolants. Properties of a medium are utilized which determine the quality of this medium, such as lowering the values of coefficients of friction. These properties are subject to internal and external influences, such as aging by light, air, operating temperature, stresses due to changes in temperature, impurities, etc. Proper operation requires a minimum quality of the medium, which must be replaced when the quality falls below this minimum. In practice the medium is replaced after expiration of a predetermined period of time or period of operation.
To the extent that sensors for obtaining parameters determining the quality of a medium have been disclosed in the art, it is a disadvantage that the output signal of these sensors also exhibits a high dependence on the temperature of the medium. If an attempt is made to eliminate this temperature dependence by measuring it through measurement of the parameter determining quality in the cold operating state, it is a disadvantage that this cold state does not as a rule correspond to the actual operating state in which the quality of the medium is a decisive factor.
DE 41 31 969 A1 discloses a lubricant oil monitoring device which records the parameters of pressure, temperature, and viscosity of a lubricant oil in situ. The proportion of lubrication-relevant long-chain molecules in relation to the proportion of the molecules already “used,” and accordingly the viscosity of the lubricant oil, are determined from measurement of the relative permittivity. The data required for this purpose is made available from the experimentally determined relationship between the relative permittivity and the technical lubricity of the oil in a storage unit.
DE 197 06 486 A1 discloses a device and a process for determining the state of aging of liquid media. At least one parameter of state of the liquid medium is determined during a first period, during which the liquid medium is in the initial state, and during at least a second period subsequent in time. The two states as determined are compared with each other. The state of the liquid medium is determined from the result of this comparison.
DE 198 50 799 discloses a sensor configuration for determining physical properties of liquids. Surface waves are excited and detected by electroacoustic converters on the polished surface of a substrate plate of uniform thickness of a piezoelectric material. The viscosity of the medium to be studied may be found from the propagation characteristic of the surface waves.
DE 101 08 576 A1 discloses a process and a device for temperature compensation of a piezoelectric device used as an actuator, for example, as positioning or drive device of a valve control system.